[Public-list] Tabernacle

Thu Dec 15 19:40:05 PST 2005

Lee;
One of the joys of being in the Great Lakes Alberg Association is that we have all 6 of
Alberg's designs possible within the Association and while we do not have any members
with either the 34 or 35, there are both in the area. I have an Alberg 22 in the shed
right now,  and help with a second one for stepping/ unstepping the mast spring and fall.
The two I have had experience with are as Gord suggests for the Shark, (another Canadian
boat) with the bolt on the deck plates and a stub fitting on the base of the mast with a
groove that straddles the bolt. In fact, the mast does pivot on the bolt that is held in
the deck plate. If the set-up on the wooden 22s was different then so be it, as I believe
they were built in the NE part of the US. The wooden 22 was drawn about 3 or 4 years
before the fibreglass on and the drawings are in the Peabody Museum if you are
interested. All of the fibreglass ones were built in Canada, with 3 makers at various
times. As Gord says, if it gets off line going up, its coming down. As for the shrounds
and the pivot points, all that happens if they are raised while connected  is that the
turnbuckles just bend, (replacement time) so the easy way to get around that is to attach
the side opposite the wharf and have a third person guy the side closest to him with a
halyard.
    There are a couple of major differences between a 22 and 30 though. The first is that
the mast for a 22 weighs about 50 lbs and is about 26 feet long and the mast for the 30
with 2 winches on it is about 200 lbs and 35 feet long. With both the added weight and
extra leverage, raising an AL30 mast in a tabernacle is just plain dangerous because the
one end is held so there is maximum leverage on this system with the fulcrum at one end.
The leverage changes considerably with the lift (fulcrum) at the spreaders. With 2
halyard winches it is almost balanced.
    The second thing that has not been considered is the horizontal loading. Most
designers do not like a 2 point mast ( deck stepped)  but make the compromise so that it
is more easily stepped than a single point mast (stepped on the keelson) as in the AL37.
The flange that is on the AL30 holds the base of the mast for any horizontal loading and
for 360 degrees.  Once on a tabernacle, this support is all lost for every direction
because the bearing surface area is so reduced for fore and aft movement or eliminated
for side to side movement. As well, there is at least one big hole in the mast, and there
had better be a compression collar within to avoid overtightening that nut on the bolt
too.
    As well, the added height from the deck point  for a 35 foot stick compared to a 26
foot mast changes the moments when in the wind, particularly if you are sailing on the
ocean where the wind might be above the surface of the water . Sail area is a third
factor to consider when comparing a 22 to a 30. While Allan Nye sold 22s with a150
foresail, they do not measure to that  and cannot be trimmed properly because the track
does not go far enough aft to flatten the draft of the 150. Most were 135 - 138 from what
I have seen measured.
    None of these discussions has mentined shock loading but if there is any choppy sea
that you are sailing in  whether under sail or not, the shock loading on the bolts is
much greater on the mast , again because the only bearing surface it the area tangent to
the bolt. That hole will not stay round for long.
    I have had experience with 22 masts getting out of control and falling and the A
frame out of column while lifting on the 30. Both are scary situations and to be avoided.
The thought of the mast coming down on the 30 or on somebody or on mooring facilities is
one of severe injury or big damage. If the rig  were to fail while in a tabernacle,while
under sail, the chances  that the  loss  would be complete would be higher than what you
now have because flat plates and holes in the mast offer small resistance to tortional
forces, compared to the plate we now have. Why people want to out design designers
without full consideration of the forces involved , particularly when the system is
compromised to begin with , is always a question I have. If you want to make things
stronger, then go to a keelson stepped mast with partners.
    I have an A frame that is 22 feet on the legs and there is a description of one on
this website that works well. I find that it works better if there is some guying (2 per
side) at the joints to a solid base off the boat to prevent the legs from getting out of
column. I much prefer a crane though for either a 22 or 30.
Don #528

FINNUS505 at aol.com wrote:

>
> Tabernacle; non-choir kind; it can be done. Some thoughts. :)
>
> The Alberg 22 had a very interesting variation in it's tabernacle that  I
> thought was an improvement over the traditional captive bolt design. Instead  of
> having the mast pivot on a bolt that was captive in the mast and the  deck
> part of the tabernacle, which would tear up the deck if the mast moved too  much
> from side to side while raising or lowering, the bolt was captive in the
> mast, but it rode in a sort of saddle, a groove in the deck plate of the
> tabernacle that kept the pin from moving for and aft, but allowed the pin to  move up
> and down, so that either end could raise significantly if the mast moved  from
> side to side, without the bolt coming out of the groove.
>
> Another article I once read about making it easier to raise and lower a  mast
> in a tabernacle addressed the problem of the shrouds being slack while the
> mast was going up until the mast was nearly vertical. The problem is with a
> cabin top stepped mast, the vertical pivot point of the tabernacle bolt is above
>  the vertical pivot point of the shroud attachment at the chainplate. This
> guy  dealt with the problem by having a ring put into the upper shroud at the
> height  of the mast step pivot point. Wires went from the ring down to the for
> and aft  lower shrouds chainplates, making a triangle, with the apex being the
> ring.  This, you remember, is at the height of the tabernacle pivot pin. As
> the mast  goes up or down, the shroud tension remains constant.
>
> Granted, for a boat as large as an A30, that ring would have to be very
> strong, but if so, the system would definitely help. Maybe nicopressing the two
> wires that lead to the lower shroud chainplates to the upper shroud at the
> height of the mast step pivot point would be a better way to do it on a big
> boat. Once the mast is stabilized from swinging side to side, rigging a pole on
> the for face of the mast to get mechanical advantage to hold the weight of the
> mast while raising or lowering becomes so much easier.
>
> The tradtional way of stepping a mast without a crane is to use mast
> 'scissors'. For an A30, I would imagine you would need 2, 20 foot long beams, at
> least 4 or five inches in diameter I would think, that had their bases fixed in
> position at the chainplates, and their apex's joined at the top. Lines from
> the  bow and stern to the apex would keep the scissors from swinging for and
> aft. A  block and tackle from the apex to a bridle around the mast at the
> spreaders  would lift the mast up, and the heel of the mast would be swung on to the
> step,  or the mast could be raised by pivoting over a tabernacle. Cumbersome.
> :)
>
> Interesting problem to ponder, when it's 20 degrees outside. :)
>
> Lee
> Stargazer #255
>
>
>
> In a message dated 12/15/2005 12:40:00 PM Eastern Standard Time,
> mainstay at csolve.net writes:
>
> Hi Bob -
>
> A tabernacle is a type of mast step.  It involves two fairly  massively
> constructed cheeks, port and starboard that in our boats would be  approx. a
> foot or more high.  The whole assembly would act as a hinge  allowing the
> mast to be swung down aft.
>
> Picture the cheeks as having  a base with vertical plates welded at 90
> degrees and set up so that the are  just far enough apart so that the mast
> will stand between them.  Then  picture the mast as having a hole drilled
> through it for something like a  1" diameter pin.  The pin would go through
> similar holes near the top  of the tabernacle cheeks near the top...in my
> example that might be about  10" above the heel of the mast.  When assembled,
> the mast would be  held with its heel slightly above the old step, which is
> now the base of  the tabernacle.
>
> If you were to want to raise or lower the stick without  recourse to a crane,
> you would be able to run a halyard forward to the bow,  then release the
> forestay and the forward lower shrouds.  By easing  the halyard you can lower
> your own mast, the base pivoting on the  tabernacle pin.  You can raise the
> mast the same way.
>
> Sounds  great...
>
> But.  If any sideways movement on the part of the mast  happens on the way up
> or down, you can either tear the tabernacle out of  your cabin top or bend
> the mast (imagine the leverage).  Or, imagine a  dismasting; you stand a good
> chance of tearing the tabernacle and a chunk  of the cabin top out of the
> boat instead of just losing the rigging  companont that failed.  And, I think
> they are ugly, but that is my  problem!
>
> I think that they are terrific for small boats that raise and  lower their
> sticks often, and for motorsailers with stumpy rigs on boats  that go under
> bridges often... I think that for boats like ours they open  more potential
> problems than they solve.
>
> Bye the way - did you want  to see pictures of my vang, and anchoring tackle
> setup? Did I ever send you  photos?  I just found some.
>
> Gord
>
> >  Gord,
> >
> > Besides being in the choir, what is a  tabernacle?
> >
> > -bob
> >
>
> _______________________________________________
> These businesses support your Association:
> http://www.alberg30.org/store/A30supporters.html
> Please support them.
> _______________________________________________
> Public-list mailing list
> Public-list at alberg30.org
> http://alberg30.org/mailman/listinfo/public-list

 1134704405.0